In the prior art, a motor speed detecting device comprises a coded disc composed of a glass plate mounted on the rotational shaft of the motor, and as an opaque portion of the coded disc passes between a light source and a light sensor, an electrical signal is obtained from the light sensor. The electric signal is then amplified to shape the waveform, thus providing a series of pulses in response to the rotational speed of the shaft. Usually, this device outputs two types of pulses, that is to say, one pulse per one revolution of the coded disc and a plurality of pulses per one revolution of the coded disc. The device counts the number per hour of these pulses, and thus detects the rotational speed of the motor.
There is an unavoidable problem wherein, when the detected rotational shaft is hollow, the above-described speed detection device cannot be applied. When a motor is used for driving a machine tool such as a lathe and the like, it is necessary to render a drive rotational shaft hollow and to make a cylindrical workpiece pass therethrough, on the basis of the mechanism for delivering the workpiece. In such a case, the apparatus using the coded disc cannot be adapted from the structural point of view, although it is recognized that the interference from other sources of light considerably affects for the worse the apparatus using the coded disc, and there remains a problem in that the precision of the device cannot always be maintained.
The present invention is intended to solve the problems described in the above prior art devices.